Support UIAccessibilityPerformEscape

[e-sung]

Summary

Fix https://github.com/slackhq/PanModal/issues/173 by responding to accessibilityPerformEscape in "PanContainerView"

Please suggest more appropriate place to respond to the gesture if there's any! 😄

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've written tests to cover the new code and functionality included in this PR.

[CLAassistant]

All committers have signed the CLA.